1. Field of the Invention
This invention relates to fluoroscopic imaging and more specifically to reduction in patient X-ray dosage during imaging.
2. Description of Related Art
An X-ray procedure, known as fluoroscopy, creates a series of internal images of a subject. Conventional pulsed systems produce each image by transmitting an X-ray pulse or other ionizing radiation from one side of the subject and detecting the transmitted radiation or shadow at an opposite side of the subject. The intensity of an X-ray radiation beam can be described by the following equation: EQU J=.intg.J.sub.o (E)e.sup.-.intg..mu.(x,E)dx dE
from p. 103 of Imaging Systems for Medical Diagnostics by Erich Krestel, Siemens Aktiengesellschaft, Berlin and Munich, where E is the quantum energy of the X-ray photons, J.sub.o (E) is the intensity at energy E of an incident X-ray beam, .mu.(x,E) is the linear attenuation constant which changes along a direction of the ray x, and .mu.(x,E) changes with photon energy E.
Different tissues exhibit different linear attenuation as a function of X-ray photon energy E, thereby exhibiting different X-ray beam intensities J after transmission through the tissue. Adjusting the X-ray photon energy, therefore, can change the relative X-ray beam intensifies as they pass through different tissue types, leading to increased contrast in an image.
The difference in intensity between the incident X-ray radiation J.sub.o and the transmitted intensity J is proportional to the dose absorbed by the subject being imaged. Compton scattering and photoelectric absorption account for the majority of the energy absorbed by the subject in the spectrum used for conventional X-ray imaging as described on p. 27 of Medical Imaging Systems by Albert Macovski, 1983 Prentice-Hall, Engelwood Cliffs, N.J. 07632.
In fluoroscopic systems, the radiation is pulsed at a rate to produce a continuous sequence of images, causing the dosage to become quite large. Fluoroscopy is commonly used in order to position a catheter or similar invasive device inside a subject. Since these procedures may take a long time, the acquired radiation accumulates to a large total dose. A primary goal of diagnostic and interventional X-ray fluoroscopic procedures is to provide an accurate diagnosis while reducing the dose received by the subject and medical staff.
Attempts have been made to reduce dose absorbed by the subject and medical staff during fluoroscopic procedures. These attempts can be classified into three categories:
(1) mechanical redesign of elements of an X-ray system such as the X-ray grid, grid cover, scintillator, table top, cassette front etc. to reduce scattering; PA1 (2) the use of protective gear (e.g., gloves and glasses, although the use of lead gloves hampers the ability to perform the fine movements necessary for catheter placement ); and PA1 (3) control of X-ray tube parameters. PA1 a) the X-ray tube voltage, which affects the photon energy of the X-rays; PA1 b) the filament current I.sub.fil, which affects the rate of emission of X-ray photons; PA1 c) the pulse duration T; and PA1 d) the pulse rate.
The X-ray tube parameters that may be varied to reduce X-ray dosage include the following:
Reduction of the filament current or the pulse duration has the effect of decreasing the exposure in each frame but at the cost of diminished image quality. The image quality is dependent on the total photon count per unit area, referred to herein as "photon count". The photon count is equal to the product of the photon rate (determined by filament current I.sub.fil) and the pulse duration T.
Pulse duration T has been reduced to limit the radiation dose as described in Effect of Pulsed Progressive Fluoroscopy on Reduction of Radiation Dose in the Cardiac Catheterization Laboratory, by D. Holmes, M. Wondrow, J. Gray, R. Vetter, J. Fellows, and P. Julsrud, Journal American College of Cardiology, vol. 15, no. 1, pp. 159-162, January 1990 and hereby incorporated by reference.
Imaging by reduced pulse rate has the advantage of maintaining the important diagnostic signal at its original high contrast level for a given dosage, but does not collect as many frames. However, the fixed rate reduction methods produce visible jerky motion artifacts. These artifacts may also introduce time delays between a physician's actions and viewed results (e.g., moving a catheter or injecting radio-opaque dye).
A technique for imaging using reduced pulse rates triggered by the subject's organ activity was disclosed in U.S. patent application "Fluoroscopic Method with Reduced X-Ray Dosage" Ser. No. 07/810,341 by Fathy F. Yassa, Aiman A. Abdel-Malek, John J. Bloomer, Chukka Srinivas filed Dec. 9, 1991 now U.S. Pat. No. 5,224,141 the present assignee and hereby incorporated by reference. Although this technique reduces dosage by reducing the pulse rate, it does not adjust the power transmitted by the X-ray source which may further reduce dose.
Incorrectly reducing the power transmitted by the X-ray source may lead to poor quality images with reduced diagnostic content--the image may be characterized by global graininess and low contrast about important features such as the catheter, balloon, vessel boundaries, etc. Attempts to improve signal-to-noise (S/N) ratio via noise reduction filters affect the overall image quality by averaging-out the noise contribution and result in the image being of questionable value since the diagnostic information is less exact at lower doses than at higher doses.
The X-ray tube voltage and current necessary to produce a high quality image also depend on the area of the body under study. It is well known that different tissue types attenuate X-rays differently. For example bone is quite dense, requiring high-energy X-ray photons for penetration, while fat is quite transparent to high-energy photons. Fat requires lower-energy X-rays to retrieve an image with good definition of the embedded features (e.g., contrast).
Since conventional fluoroscopy systems may incorrectly calculate X-ray tube voltage and photon count, subjects may be exposed to more radiation than is necessary, or the images produced may be grainy and lack desired contrast.
Currently, there is a need to determine the required X-ray tube voltage and photon count accurately so as to produce a high quality image, while also minimizing the X-ray dose to the subject.